The effects of mineralocorticoids, arginine vasopressin, and bradykinin on isolated perfused cortical collecting ducts from rats are being investigated. Chronic deoxycorticosterone treatment of the rats increases sodium absorption and potassium secretion by the collecting ducts in vitro. Addition of arginine vasopressin to the bath in vitro increases both the sodium and potassium transport above the deoxycorticosterone-stimulated levels. Addition of bradykinin to the bath in vitro inhibited the sodium absorption, but did not significantly affect potassium transport.